In recent years, as various kinds of movable bodies, vehicles such as a motorcycle, an automobile, and the like have been electrified after development and practical realization of an electric bicycle, an electric cart, and the like. Specifically, while replacing an automobile that utilizes an internal combustion engine as its driving power source, a hybrid automobile, which utilizes a gasoline engine and an electric motor as its driving power source, an electric automobile, which utilizes, as its driving power source, an electric motor that operates with a domestic electric power source or a battery to be charged through an electric charger installed at a gas station or an electric power supply station, and a fuel cell automobile, which travels while generating electricity through a fuel cell that utilizes hydrogen gas or the like, as a fuel, have sequentially been developed; a hybrid automobile and an electric automobile have already been put to practical use and have started to spread.
In the case of each of a gasoline vehicle, a diesel vehicle, and a motorcycle (hereinafter, described as “a conventional automobile and the like”) that each utilize a conventional internal combustion engine as the driving power source therefor, an engine sound and an exhaust sound, which are emitted by the driving power source itself, and road noise or the like during its travel are generated; therefore, a pedestrian walking in a town, a cyclist, or the like can be aware of approach of a vehicle through an engine sound, an exhaust sound, or the like. However, because during a low-speed travel, a hybrid automobile travels mainly by means of not an engine but an electric motor, no engine sound or exhaust sound is generated, and an electric automobile, a fuel cell automobile, and the like each travel by means of an electric motor in the whole driving region; thus, any one of these automobiles is extremely high-quietness electric vehicle. However, a pedestrian or a cyclist in the vicinity of such a high-quietness electric vehicle cannot perceive through a sound an approach of the electric vehicle such as a hybrid automobile, an electric automobile, or a fuel cell automobile that travels by means of a less-sound-noise and high-quietness electric motor; therefore, this may become the cause of a minor collision between the high-quietness electric vehicle and the pedestrian or the like.
Accordingly, in order to solve the foregoing problem in which the quietness, which is originally a merit of each of a hybrid automobile, a fuel cell automobile, and an electric automobile, there have been proposed various kinds of approaching vehicle audible systems, other than a horn that is provided in a conventional automobile or the like and sounds an alarm in accordance with the will of a driver, that as to notify a pedestrian in the vicinity of a reference vehicle of the existence of the reference vehicle.
Some of the approaching vehicle audible systems make a sound that resembles a conventional engine sound, as a notification sound. For example, Patent Document 1 discloses a technology that stores a matching table including an accelerator opening degree and the virtual engine rotation speed based on the accelerator opening degree so as to obtain a virtual engine rotation speed corresponding to an accelerator opening degree and an elapsed time.
Patent Document 2 discloses that data on a generated sound is stored as digital data in units of a sound pressure waveform having a length corresponding to the time during which the crank shaft rotates in one combustion cycle and that when at a rapid acceleration mode, the order of reading a plurality of digital data pieces to be reproduced is controlled for a delay of the engine rotation speed. In other words, in accordance with the values of the accelerator opening degree and the engine rotation speed, the digital data pieces to be reproduced are selected.